Tuesday, 7 April 2015

The following blog post was really scheduled for a couple of
months ago but I just never got round to publishing it. Having said all that,
the science is still cutting edge and just as fascinating as it ever was so I
hope you find it of interest.

Forelimb reduction
in theropod dinosaurs has long since fascinated. It is a constant source of
interest that some of the largest carnivorous dinosaurs reduced the size of
their forelimbs to such a degree that it would appear that they were almost
becoming vestigial or, at the very least, were only capable of speculative
rudimentary function. Chief amongst palaeontologists looking at this enduring
mystery is Sara Burch of Ohio University.

To tackle these enduring questions, Burch approached the
issue in quite a unique way – firstly by constructing a model of the ancestral
forelimb muscle arrangement highlighting any associated plesiomorphies. Then,
by looking at the phylogenetic signals generated by examining the evolutionary
processes in various theropod lineages, Burch has come up with some useful
data.

Both allometric and evolutionary trends suggest that there
is no evidence for a general reduction in forelimb size throughout Theropoda. A
clade by clade study also revealed interesting myological trends – particularly
in Abelisauridae and Tyrannosauridae. Abelisaurids display a quite unique
morphology and yet, despite their forelimbs appearing to be useless, the study
reveals them to be still functional. It is unlikely that they were capable of
much, however, and it may be that the forelimbs were used for sexual
stimulation – speculative, of course, but not the first time this has been
suggested for abelisaurids or, indeed, tyrannosaurids.

Tyrannosaurids themselves have robust and muscular arms and
one suggestion to try and describe a particular use for these forelimbs was
that it enabled the animal to push itself up from the ground when required.
This particular hypothesis was modelled accordingly with the outcome that there
is no justifiable support for the theory. But what about possibly using their
arms for grasping or holding prey? Muscle correlation concludes that this was
indeed possible supporting the evolutionary trend that reduction in forelimb
size is not necessarily about vestigiality but rather to satisfy an
evolutionary demand. As with abelisaurids, the theory that tyrannosaurids may
have used their forelimbs intraspecifically remains untestable.

Pterosaurs currently remain the biggest flying animals of
all time and yet was there a size limit that dictated how big a pterosaur could
be and still manage to take off, fly and land? Colin Palmer, of the University
of Bristol, and Mike Habib of the University of Southern California, have been
addressing these very issues.

Azhdarchids were the
giants of the pterosaur world but estimates of their weight and mass vary considerably.
For example, Chatterjee & Templin (2004) suggested a body weight in the
region of 70Kg whilst Mark Witton (2008) and Witton and Habib (2014), more
realistically for an animal approaching the size of a giraffe, estimated 260Kg.
The general consensus is that the latter is probably correct.

As for wingspan it appears that the most common sizes vary
from 5 metres to 9 metres with an upper limit, perhaps, of twelve metres – true
giants. Computer models were generated for pterosaurs with 6, 9 and 12 metre
wingspans which were as structurally and aerodynamically as accurate as the
fossil record allows bearing in mind most of the data is derived primarily from
ornithocheirids.

For an azhdarchid to remain in the air depends, essentially,
on the available power that is generated by the muscles. The model indicates
that these pterosaurs were capable of generating sufficient power to maintain
station once airborne and, interestingly, that the ability to stay up does not
limit the size of the pterosaur. In other words, pterosaur maximum size is
limited by the amount of tensile stress generated but is not limited by size
alone. Therefore, it is quite feasible for a pterosaur with a 12 metre wingspan
to remain airborne.

What about landing? A large extant bird requires a stopping
distance of around 4 metres per second and the models predict that large
pterosaurs also fall within this range and the authors point out that the
robust hind limbs of azhdarchids make for a pretty sturdy undercarriage so it
seems possible that a pterosaur with a 12 metre wingspan could also land
safely.

But could a pterosaur of these proportions generate the
sufficient power, technique and thrust to launch itself into the air? It is safe to assume that pterosaurs utilised
both forelimb and hind limb musculature to achieve take off and the computer
models generated for this research reflect this although, interestingly, 80 to
90% of the required take-off thrust for both birds and pterosaurs is developed
from the hind limbs.

Anhanguera forelimb musculature - from Witton & Habib 2010

Taking other factors into account such as oscillation and
both launch speed and height indicates that it is problematic for pterosaurs to
achieve take off as they get bigger since they could not evolve a sufficient
muscular array that would be structurally efficient for take-off. The results
suggest that we can predict with confidence that pterosaurs with a 9 metre
wingspan could successfully take off but with a lesser degree of confidence
that those with wingspans between 9.5 and 11.5 metres could achieve launch.

Thus this research is indicative that 12 metres is currently
the absolute wingspan limit for pterosaurs – which we know of. This is really
interesting research and it would be fascinating now to see the response if
another pterosaur is found with perhaps a 14 or 15 metre wingspan. Unlikely of
course but that would really put the fat into the fire because who would not
want to know how that would have been achievable and what other factors would
need to be considered.

Theropods come in all shapes and sizes and we are familiar
with the various sizes and morphological differences in their teeth but what
about the actual mandible itself? Is theropod jaw form representative of
function or, indeed, is the taxanomic, morphological and functional diversity
of the lower jaw a predictor of functional and biomechanical diversity? Emily
Rayfield, of the University of Bristol, and her colleagues have been asking
these very questions.

A sample size of 103 specimens was analysed using a
combination of geometric morphometry and biomechanical metrics. The authors
point out that the sample size, although relatively broad, is still nowhere
near large enough but included a diverse group sampling which included, amongst
others, non-tetanuran theropods, non-maniraptoran tetanurans and
maniraptorformes themselves.

So do theropods that feed on different things have different
shaped jaws? Apparently not since the authors discerned no particular signal
from this analysis. What about functional traits in theropod jaws? The authors recognised 19 traits in 68 of the
taxa that are related to jaw robustibility and the enlargement of the coronoid
process which contribute to the overall shape and biomechanical variation. As a
side note it is worth pointing out the oviraptorosaurs, although included in
the analysis, are quite distinct from this overall grouping and were obviously
doing something very different.

Theropods that are herbivorous or exhibit omnivorous
tendencies display different shaped jaws to traditional carnivorous types which
is to be expected. Theropod mandibular evolution throughout the Mesozoic
suggests that there is likely to be a link between form and function and this
is supported by phylogenetics which does indeed exert a strong signal. However,
this is probably exaggerated by the fact that Maniraptora filled many different
ecological niches from the Late Jurassic onward which would have demanded more
morphological variation. The overall link, therefore, between morphological
change and functional diversity is
tenuous at best and suggests that perhaps the shape of the jaw does not always
necessarily reflect the evolved and/or derived state of different theropod jaw
mechanics.

Palmer, C. & Habib, MB. 2014. All time giants of the
air: new approaches to calculating the limits to the size of pterosaurs. Journal
of Vertebrate Paleontology, SVP Program and Abstracts Book, 2014, pp200-201.

Witton MP, Habib MB (2010) On the Size and Flight Diversity
of Giant Pterosaurs, the Use of Birds as Pterosaur Analogues and Comments on
Pterosaur Flightlessness. PLoS ONE
5(11): e13982. doi:10.1371/journal.pone.0013982

Saturday, 24 January 2015

Tyrannosaurids are best known from the Late Cretaceous
deposits of North America and Asia and, it is fair to say, that we have a very
reasonable fossil dataset from both continents. And it is also worth
reiterating that our understanding of tyrannosaurid paleobiology and phylogeny
is one of the fastest moving disciplines in palaeontology. This should surprise
nobody as they are amongst the most interesting and impressive of all dinosaurs
and, as such, will always inspire a fervent following – myself included.

In recent years we have seen the arrival of new
tyrannosaurids such as Teratophoneus,
Bistahieversor, Lythronax, Nanuqsaurus
and Zhuchengtyrannus - and there are
more to come. For instance we already know that there is a second species of Daspletosaurus inthe pipeline for publication (Carr & Varricchio 2014) which I
will, obviously, blog about in due course and there are one or two other
surprises coming in the future as well. Even the relatively unknown Alioramus has undergone an extensive
redescription recently due to new fossils being found (eg Brusatte et al 2009, Brusatte et al 2012, and Lü et al 2014) and we have a much greater understanding of the so
called “narrow snouts” than we ever
did.

So we appear to know a lot about them but, in real terms, we
still understand very little and interpretation of the fossils is often subject
to debate. Tyrannosaurids of all taxa are actually very similar animals and, to
be fair to all the palaeontologists concerned, it is easy to see why there are
so many discussions when so many of the anatomical details are often slight.
One man’s new taxon is another man’s different ontogentic stage of a current
taxon. Morphological change throughout ontogeny is, without a doubt, the
biggest single determining factor in dinosaur phylogeny.

The origins of Tyrannosauridae and its distribution is also
a matter of contention - some people prefer an Asian origin whilst others, by
way of example, now favour a radiation that evolved in southern Laramidia. I
favour neither nor disregard either and I believe that it is fair to say that
there were obviously multiple migration events with taxa of different groups from both Asia and North America
moving back and forth across Beringia which, unsurprisingly, may also give strange
or false phylogenetic signals on occasion.

The earliest true tyrannosaurid is undoubtedly yet still to
be discovered and we do not have the best dinosaurian fossil record beyond the
Campanian and it would be terrific if there were fossils to be found in the
Santonian and probably the Coniacian too – but where would be anyone’s guess.

So with all of this in mind it seems that, despite the
relative wealth of fossil material that we have of tyrannosaurids, we are in
need of a hell of a lot more to increase the dataset. So it can cause
consternation amongst palaeontologists’ when what may be important specimens to
science are sold off at auction. Thomas Carr recently highlighted a number of Tyrannosaurus specimens that are held in
private hands and, as such, are off limits to professional researchers.

This is unfortunate. There are many who would say that since
there are multiple specimens of Tyrannosaurus
already in institutions then what is the point of having more? Without
increasing the aforementioned dataset then we will continue to have large gaps
in our knowledge and never be able to fully understand the paleobiology,
morphology and ontogeny of these majestic creatures.

And just to amplify how important it is to have a large
sample we need look no further than by using another Hell Creek giant, Triceratops, as an example. There are
hundreds of skulls of this animal – hundreds!
In fact nobody is really sure just how many there are but when compared
with good skulls of Tyrannosaurus
(perhaps 20?) then it is amazing that there is still debate regarding the
amount of Triceratops species there
are, whether Torosaurus is the adult
morph of Triceratops and, indeed, the
amazing morphological change that is displayed throughout ontogeny. Indeed, this is just as applicable to other
ceratopsians as well.

Hadrosaurs are another extremely well known group and,
although there are many interpretations of their fossils, they do not appear to
induce the often emotional discourse that follows tyrannosaurids and, to a
lesser degree, ceratopsians. And we have not even got to the world of
dromaeosaurids and troodontids for which there is a dearth of GOOD fossils but
no shortage of opinion and speculation.

Therefore tyrannosaurid specimens are important – all of
them and, as we can see, this relates to all clades of dinosaurs as well. So
when I learnt that a rather stunning tyrannosaurid maxilla and premaxilla from
the Republic of Kazakhstan had come to the surface I became rather excited.
There is a diverse fauna of Cretaceous dinosaurs that are recognised from the
ex-soviet republic including Hadrosauridae, Ankylosauridae, Sauropoda and
Tyrannosauridae amongst others.

Some remains can be locally abundant – hadrosaurs in
particular whilst others are known from rather scrappy and indeterminate
remains. Averianov et al (2012) reported on a right dentary from an unspecified
tyrannosaurine that was recovered from Late Cretaceous rocks in Kara-Cheku
which is located in Almaty Province. This was collected back in 1950 and
labelled as belonging to Tyrannosaurus
sp. – actually delightfully labelled as “Tirannosaurus”.

Without decrying the specimen too much, it is not the most
aesthetically pleasing example but it is, never the less, an important fossil
and the authors were able to confirm its assignation to a tyrannosaurine
tyrannosaurid. The fact that this is one of the better tyrannosaur fossils from
Kazakhstan again reinforces how poor the record is from this country and we
hope that more specimens will be discovered in due course.

So where on earth did this new specimen come from? The
documentation, such as it is, is fairly vague. It was collected in, and, I
quote “…the second half of the previous
century...” from the Zhetysu region of Eastern Kazakhstan. If correct then
this is certainly of interest since the eastern region has been considered
fairly non-productive for Cretaceous vertebrates (Averianov et al 2012) so one wonders about the
collecting locality of this specimen. The specimen itself is striking and
represents a right maxilla with its accompanying premaxillary attached and is
approximately 360mm in length albeit with about <>5% restoration . As you
can see, there are at least 12 maxillary teeth in situ and it appears that the
four premaxillary teeth are also present and correct. All the teeth are genuine
and in position as found.

The maxilla displays a gently convex anterior edge but the
posterior appears not to be as acutely convex as is the case in Zhuchengtyrannus (Hone et al 2011). The 12 maxillary alveoli is
the typical count for both Tyrannosaurus
and Tarbosaurus. The rugose surface
is also common in derived tyrannosaurids although this sculpting is more
heavily defined in mature specimens – which this does not appear to be. The subnarial foramen is readily apparent but
detail is hard to define. But then this is the real point about this specimen.
In comparison with known tyrannosaurid material from Kazakhstan this specimen
is actually quite spectacular and really important and we should all look
forward to its description.

But there is a disturbing caveat here. The only reason I am
aware of this specimen is that I was tipped off that it was going to auction
and all this “detail” I have is only because I referred to the auction website
and inspected the images. I do not intend to go into detail here but the specimen
was sold into private hands for a not inconsiderate amount of money and these
pictures I am sharing with you today may very well be the only public record of
this specimen now that it has probably disappeared forever.

Now it may be that this specimen is not from Kazakhstan at
all and when you have a specimen with so little recorded history then it is a little
bit of a punt whether the specimen appears to be what it says it is. Regardless
of this, and in the correct repository, careful research should be able to
address the taxanomic issues and, perhaps, where the fossil originated from. If
it is from Kazakhstan then it is important, important, IMPORTANT!

It does not matter what your opinion is on the buying and
selling of fossils – for there are many but one thing remains absolutely certain.
It is awful that important specimens, like this tyrannosaurid maxilla, are lost
forever in the eternal blackness that is private ownership never to see the
light of day and I wish that we could convince these people to donate these
specimens to the correct repositories so that they are available for research
and so that we can, indeed, fill in these missing gaps in our evolutionary
history.

References

Brusatte SL, Carr TD, Erickson GM, Bever GS, Norell MA
(2009). A long-snouted, multihorned tyrannosaurid from the Late Cretaceous of
Mongolia. Proceedings of the National Academy of Sciences of the United
States of America, 106(41), 17261–6.
doi:10.1073/pnas.0906911106

Brusatte SL, Carr TD, Norell MA (2012). The osteology of Alioramus, a gracile and long-snouted
tyrannosaurid (Dinosauria: Theropoda) from the Late Cretaceous of Mongolia. Bulletin of theAmerican Museum of Natural History Number 366, 197 pp., 82 figures, 11 tables Issued February 29, 2012.

Carr, T & Varricchio, D. 2014. A new species of Daspletosaurus from the Upper Two
Medicine Formation (Late Campanian, Cretaceous) of Montana and evidence for
anagenesis in tyrannosaurine evolution. Journal of Vertebrate Paleontology,
SVP Program and Abstracts Book, 2014, pp103-104.

Search This Blog

Saurian is.....

I am an avocational vertebrate palaeontologist with a particular interest in tyrannosaurids. Fossil preparation, conservation and research also feature high on my agenda.
Currently involved in the day to day running of a progressive and exciting research group - we are hopeful of making a positive contribution to our science.